1. Field of Invention
This invention relates to a hydraulically operated power steering system for use in automotive vehicles and more particularly to an improvement in a hydraulically operated reaction mechanism for use in a power steering system which comprises a reciprocating-type hydraulically operated cylinder unit for moving a steering linkage and having first and second chambers, a reservoir containing hydraulic fluid therein, a hydraulic pump feeding the hydraulic fluid in the reservoir to the cylinder unit, a control valve unit including a line coupled to communicate with the hydraulic pump, a line coupled to communicate with the reservoir, a valve cylinder having passages coupled between the first and second chambers to provide fluid communication therebetween, and a valve spool slidably arranged in the valve cylinder and reciprocatingly movable in response to rotation of an input shaft, wherein the hydraulic fluid from the hydraulic pump is introduced to the first hydraulic chamber upon rotation of the input shaft in one direction while the hydraulic fluid in the second hydraulic chamber is discharged to the reservoir, and wherein the hydraulic fluid from the hydraulic pump is introduced into the second hydraulic chamber upon rotation of the input shaft in the other direction while the hydraulic fluid in the first hydraulic chamber is discharged to the reservoir.
2. Description of the Prior Art
A conventional power steering system of this type is disclosed in Japanese patent publication No. Sho 49-1809. The disclosed system comprises a valve cylinder, a valve spool axially slidable relative thereto and having an axially extending bore, and two reaction pistons incorporated in the bore of the valve spool and non-shiftable relative to the valve cylinder. In such a system, hydraulic pressure is supplied in proportion to the application of steering force as shown in FIG. 5. When a car travels at a low speed, small steering force is required. However, if a steering force is lessened when the car travels at a low speed, such a steering force will become too small when the car travels at a high speed, whereby the maneuverability is deteriorated. In order to overcome this problem there has been proposed a system (greater hydraulic pressure is supplied upon the application of small steering force when the car travels at a low speed, whereas less hydraulic pressure is supplied upon the application of large steering force when the car travels at a high speed) wherein a variable orifice is provided to communicate reaction chambers with hydraulic chambers. With this arrangement, the characteristic curve is rendered variable, as shown in FIG. 6. However, this system is rather complicated and thus, costly.